The terahertz (THz) portion of the spectrum (i.e., frequencies greater than 1012 hertz (Hz)) represents a current frontier for multidisciplinary science and technology. Enormous potential opportunities exist in engineering, physics, material science, chemistry, biology and medicine, and particularly when using the portion of the spectrum between 0.5 and 20 THz. For example, practical applications abound in the areas of imaging, sensing and spectroscopy, such as medical imaging, industrial imaging (i.e., package inspection), homeland security, gas sensing, biological spectroscopy including bio-warfare agent detection, explosive detection and astronomy. Some of these practical applications stem from the unique ability of THz radiation to safely penetrate a wide-variety of non-conducting materials including clothing, paper, cardboard, wood, masonry, plastics, ceramics, etc.
In addition to THz time domain spectroscopy, THz generation by optical down-conversion in nonlinear optical materials has become a viable alternative way to generate THz radiation. Generating THz radiation by optical down-conversion was first demonstrated in the early 1970s, but it has become more popular recently due to the availability of reliable ultra-fast lasers. THz generation by optical down-conversion in optical parametric oscillators (OPOs) provides a good illustration of the Manley-Rowe conversion limit, where a high energy optical photon (i.e., a pump) generates a THz photon and a lower energy optical photon (i.e., an idler). However, because each pump photon can generate only one THz photon, the power conversion efficiency will be limited to less than 1% even when the photon conversion efficiency is 100% due to the lower energy of the THz photons. Therefore, the typical power conversion efficiency of THz OPOs is typically on the order of 2×10−8. Additionally, the short interaction length between the generated THz radiation and the optical pulses limits the efficiency of THz generation by ultra-fast optical pulses in nonlinear crystals. For example, the interaction length (i.e., the coherence length) is limited by the velocity mismatch between the optical pulses and the generated THz radiation due to dispersion. Thus, the optical down-conversion process is most efficient in materials having longer coherence lengths such as ZnTe, where the coherence length reaches several millimeters for an 800 nm pump laser. However, distortion leads to broadening of femtosecond optical pulses in ZnTe, which reduces the peak power conversion efficiency.
Quasi-phase matching (QPM) microstructures can effectively extend the coherence length between the generated THz radiation and the optical pulses. Thus, the development of QPM crystals, in which periodically-patterned changes in the sign of the nonlinear coefficient compensate for the wave-vector mismatch, has revolutionized many aspects of nonlinear frequency conversion. For example, by replacing the dependence on naturally occurring birefringence with the lithographically-controlled (i.e., systematically-engineered) patterning, the range of operation of the rare, well-developed nonlinear material is readily extended across its transparency range. Extremely high mixing efficiencies are attained because materials with large nonlinear coefficients are accessible and noncritical operation (i.e., propagation along a symmetry direction of the crystal) is always possible. In addition, non-uniform (lateral or longitudinal) QPM gratings allow tailoring of the tuning behavior in ways impossible in birefringently-phase-matched media.
The coherence length between the generated THz radiation and the optical pulses can be extended using QPM microstructures such as periodically-polled LiNbO3 (PPLN) and orientation-patterned GaAs (OP-GaAs). The QPM microstructures include the periodic system of domains of inverted crystal orientation. As a result, the phase of the nonlinear polarization generated by short optical pulses changes by 180° at the domain boundaries. If the domain length is equal to the coherence length, phase-matching conditions between the THz radiation and the nonlinear polarization will be restored at the domain boundaries, which extends the interaction length between the THz radiation and the optical pulses.
THz generation using OP-GaAs has been studied because of several unique characteristics of GaAs. Specifically, GaAs has high transparency in the THz spectral region, a high nonlinear coefficient and low dispersion in the near-infrared and THz spectral regions. For example, U.S. Pat. No. 7,339,718 to Vodopyanov et al. and U.S. Pat. No. 7,349,609 to Vodopyanov et al. disclose THz laser sources using OP-GaAs semiconductor crystal. The current state-of-the-art includes a near diffraction-limited THz laser source using an OP-GaAs semiconductor crystal with 1 mW average power output, an optical-to-optical conversion efficiency of 0.01% and a tunable frequency between 0.65 and 3.4 THz.
However, THz generation using an OP-GaAs semiconductor crystal is severely limited in practical applications due to free-carrier generation by high-intensity optical pulses due to two-photon absorption, which is very strong in the THz spectral range. Thus, the pump power and the choice of pump beam wavelength are practically limited. For example, in order to avoid two-photon absorption and limit losses in THz power output, the pump laser must emit at 2 μm (i.e., below the two-photon absorption edge of GaAs). High-power femtosecond lasers that emit at 2 μm, however, are not readily available, and instead are only available by custom order, which significantly increases the costs by up to $200 k, for example. In addition, an additional OPO system is required to obtain 2 μm laser emission.